High fidelity loudspeakers



July 6, 1965 w. HECHT HIGH FIDELITY LOUDSPEAKERS Filed May 51, 1962 vINVENTOR.

WILL/Al? H'C/{T United States Patent 3,193,627 HIGH FIDELITYLOUDSPEAKERS William Heclit, 11 Normandy Terrace, West Orange, NJ.

Filed May 31, 1962, Ser. No. 199,148

9 Claims. (Cl. 179-4155) This invention relates generally to highfidelity loud speakers, and particularly to an improved design for thevoice coil assemblies used in such speakers, by means of which theeffects of distortion are more effectively subdued over the entireoperating spectrum of the speakers.

An object of my invention is to provide loud speakers apparatus in whichthe forces associated with transient distortion are cancelled by themagnetic damping action of the voice coil assembly.

Another object is to provide loud speaker apparatus in which the sounddistorting effects of extraneous disturbances, such as, for example, theeffects associated with system resonances, standing waves, sympatheticvibrations, tuned ports and the like, are more effectively nullified.

Another object is to provide loud speaker apparatus exhibiting improvedconstant velocity damping characteristics over the entire operatingfrequency range thereof, while avoiding undue complication of the designof said apparatus or increase in the cost of manufacture thereof.

As a feature of my invention I provide a voice coil assembly includingthe equivalent of a short circuited conductive path, the dimensions andcomposition of which are carefully selected in association with themagnetic field intensity between the poles of the speaker magnet, toprovide critical magnetic damping at all frequencies, and thus tomaintain a constant velocity voice coil response regardless of theorigin of a distorting force.

These and other objects and features of my invention will be more fullyunderstood and appreciated when considered in relation to the followingdescription taken in connection with the accompanying drawing wherein:

FIGURE 1 is a simplified diagrammatic view of loudspeaker apparatus withreference to which my invention is explained,

FIGURE 2 is a view in perspective of a voice coil form in accordancewith my invention illustrating the circuit paths followed by electricalcurrents induced therein, and

FIGURE 3 is a view in elevation of an alternative voice coil formconfiguration in accordance with the invention.

The art of high fidelity loudspeaker design has reached a highlyadvanced state in which further progress is confined in general torelatively small design innovations. My invention relates to one suchseemingly small, but significant, innovation.

One indication of the quality of performance of a loudspeaker is itstransient response, this being simply the response to sudden or rapidlyvarying input excitation of the type commonly associated with cymbalcrashes, or the like. It is generally well known that to faithfullyreproduce such transients a loudspeaker should exhibit constantvelocity'frequency response. In other words the peak velocity of thespeaker cone fluctuations should be constant, for a given constant levelof power input, over.

manufacturer, and thus the characteristic frequencies of externallyoriginating resonances, or other extraneous velocity distortinginfluences, are generally unpredictable. It would follow therefore thatin high fidelity speakers provision should be made for the inclusion ofmeans to nullify, or to compensate for, such extraneous influences atall frequencies of interest. The problem however is to provide suchmeans without undue complication of the design, and without undueincrease in the cost of manufacture, of the speaker apparatus.

Reference is had to FIGURE 1 for an explanation of the novel techniqueby means of which I provide the requisite compensation. As showntherein, my apparatus contains the usual stationary assembly including apermenent magnet 1, having respective inner and outer pole pieceextensions 2 and 3, separated by a narrow annular recess 4 hereinafterdesignated the gap. Disposed symmetrically within the .gap are a voicecoil form 5, of generally cylindrical shape, and a voice coil winding 6which is wound on the form 5. One end of the form 5 is connected to 'thebase of a sound producing diaphragm 7, the members 4-7 therebyconstituting a movable assembly, the motion of which corresponds to theelectrical variations applied to the coil 6. The free end of the form 5extends beyond the gap, the length of this extension being greater thanthe maximum displacement of the movable assembly, so that the volume ofform material within the gap is held constant movement of the saidassembly, for reasons which are more fully discussed'below.

It is well known that for optimum efficiency the above movable assemblyshould be made as light and as precisely dimensioned as possible.Accordingly, the form 5 is usually obtained by precisely molding paper,or other lightweight moldable material having suitable structuralcharacteristics, into the required shape. As it is unfeasible to castmetal forms in the required shape, and with proper thickness, andweight, metal forms are not generally used. In certain instances howevermetal forms are used, composed of a metal having a high strength toweight ratio, such as aluminum, brass, or the like, and which serveexclusively as a support for the voice coil.

As the coil assembly must fit precisely and symmetrically into the gap,the usual practice, with respect to metal forms, is to fashion the formout of a rectangular strip of metal into a cylinder having alongitudinal split, as at 8 in FIGURE 2, and to place the form over aprecisely shaped mandrel, the diameter of the form being made to conformto the diameter of the mandrel by adjustment of the width of the split8.

Returning to the problem of transient distortion, it is generally wellknown that a properly damped moving coil system will not exhibit suchdistortion. It is also well known, however, that a voice coil which isproperly damped at one frequency will beimproperly, or ratherinsufiiciently, damped at other frequencies due to the usual variationin the electrical impedance of the coil over the operating spectrum.Mechanical damping structures being cumbersome, costly, and limited infrequency response,

the usual practice is to append additional electromagnetic dampingstructures to the moving coil structure. The latter, however, add massto the system, thus reducing the eficiency and increasing the naturalresonant frequency thereof, in addition to complicating the design ofthe system.

This then leaves the voice coil form as the only available,non-additional damping structure. Metal forms, however, are extremelythin and light structures, the thickness usually being on the order of0.005 inch, and the mass being on the order of 10 percent of that of theentire moving structure which is typically on the order of 0.02kilogram. \Because of this and also because of the apparent electricaldiscontinuity presented by the longitudinal split in the form cylinder,it is generally assumed that the electromagnetic damping action of ametal voice coil form' is negligible. I have experimentally verified theabsence of significant damping for an aluminum form by dropping asuitably loaded aluminum form of typical dimensions into acorrespondingly typical magnetic gap, and by noting the apparent absenceof any viscous drag as the form drops. The same is true of almost allmetals. Surprisingly enough, however, forms made out of pure copper orpure silver, and only these two metals, exhibited a marked degreeof'viscous damping action approaching the optimum for wide-band fre- Iquency response without overshoot, or approximately seventy percent ofcritical damping. Furthermore, loudspeakers which I have constructedcontaining copper and silver forms exhibit a marked improvement infrequency response, especially at the upper end of the frequency range,where the voice coil damping action tapers off. The compensating actionof the copper and silver forms almost completely eliminates the peak inresponse'curve associated with the behavior of the diaphragm athighfrequencies (see, for example,.L. L..Bera11ek, Acoustics, McGraw-HillBook C0,, p. 199, et seq.). f, 1 Thus it may be narrowly concluded, andthis is so claimed below as oneattribute of 'my invention, that copperand silver voice coil forms of a given size, the dimensions of which arelisted in an examplecited below, constitute almost ideal dampingstructures when operated in a magnetic field of a given intensity, alsocited below.

However," an even more general conclusion as to the nature of myinventive discovery may be drawn from the following analysis.

The forces acting on a damped vibrating system are the system in meters,and the respective first and second derivatives thereof with respect totime;

K represents the spring constant of the moving system,-

in newtons/meter; K represents the damping constant, in newtons-meter/sec.;

M is the mass" of the moving system in. kilograms; and

F is the total force acting on .the system, in newtons.

Substituting'A-sin wt for x, where w is thefrequency of vibration, inradians/sec, we have: i

At resonance, the reactive term K -.-MW vanishes.

where w is the resonant frequency.

Thus, the 'basic differential equation may be written- In a criticallydamped syst-em, the radicalof thecorresponding quadratic expression isequal to Zero. Thus For the split cylinder coil form of FIGURE 2, as

used in the system of FIGURE 1, it may be shown that inducted eddycurrents follow the pattern indicated, in FIGURE 2, by dotted linesonthe hidden surfaces of the cylinder, and by solid lines on the visiblesurfaces, one such current path being traced by the letters ABCD inFIGURE 2. The damping forces F due to eddy currents induced in the form5,are given by:

F '=BL I.

where, V V 7 I B is' the flux density (in gap 4 of FIGURE '1), inwebers/ square meter; Y p

7 Thus 4. L is the circumferential length of the form cylinder,

in meters; I represents the induced current, in amperes, given by: I=BLTv/k where:

L is the length of the gap 4, in meters;

T is the thickness of the form, in meters;

v is the velocity, as stated above; and

k is the resistivity of the form material, in ohm-meters.

7 F eL zea L L rv/k Hence, the damping ratio R, defined as the ratio ofthe actual damping force F to the critical damping force F is given by:V

, R=B L L T/ 2kMw Solving for the resistivity k:

I V V k--=B?L L T/2RMw (1) Substituting 11/2 for L,,/w in Equation 1where d is the diameter of the form cylinder, as shown in FIG- URE 1,and where his the resonant frequency in cycles/ k=B dL T/2.8Mf V (3) andEquation 1 becomes:

' k=B L L T/1.4Mw, 4)

I A typical set of'value's used in the construction of speakers, of'thetype shown in FIGURE 1, is given as follows:

I B=l Weber/sq. meter 10 gauss) d=.05 meters (2 inches) L -=.0095.meters (.375 inch) T=.00013 meters (.005 inch) M=.02 kilograms, and

f cycles/ sec.

Substituting the above values in Equation 3, we have:

Referring to any table of resistivity values, for example that given onpage 197 of the above-referenced book Acoustics, it is seenthatthere aretwo materials which approximately provide the above value ofresistivity, and all the rest do not. These two materials are silver(;0163 l0- ohm-m.), and copper (.0l72 1O- ohmm.), which verifies theabove-discussed experimental observations.

Thus it has been'shown that for the specific set of speaker constructionvalues listed above, only a coil form made out of silver or copper-willprovide optimum damping (R =0. 7), and further that for any set ofspeaker construction values, a value of resistivity k, may be deterfmined, by means of Equation 3,0r 4, above, ,such that acoil formconstructed of, a material having a resistivity value nearest to' thedetermined mum system damping.

value will provide opti- 1 An alternative coil for-m construction inaccordance with my invention is shown in FIGURE 3, wherein the ment theparallelogram, indicated by the dotted outline 11, 12, is formed into acylinder over the above-mentioned m-andrehwith the cylinder lengthwiseedges 9'and 19 preferably abutting. The coil is then wound in the mannerstated above. By thus reducing the gap 8, the induced form current, andtherefore the damping eltect, may be slightly increased, for a givenmass of form material, but at the expense of an additional design andmanufacturing complication with respect to alignment of theparallelogram edges 9 and 10.

While the invention has thus been shown and discussed in connection withspecific embodiments, it should be clearly understood that these havebeen given solely by way of example and not as limitations to the scopeof the invention as set forth in the objects thereof, and in theaccompanying claims.

Accordingly, I claim:

1. In a moving coil loudspeaker, a moving coil structure comprising:

(a) a form defining a constant velocity damping element composed ofmaterial selected from the group consisting of relatively pure copperand relatively pure silver, to be positioned in a magnetic gap,

(b) a voice coil winding on the form,

(c) a magnet having inner and outer pole piece extensions disposed todefine a magnetic gap to receive between the inner and outer pole pieceextensions the voice coil and form,

(d) a sound producing diaphragm connected to one end of the form,

(e) the form longitudinally dimensioned sufficiently greater than thelongitudinal dimension of the gap so that the free other end of the formwill be beyond the magnetic gap at maximum displacement of the voicecoil.

2. In a moving coil loudspeaker, a structure according to claim 1wherein the form provides approximately seven-tenths of the forcerequired to critically damp said moving coil structure.

3. A moving coil loudspeaker according to claim 1 in which the copperhas the approximate resistivity value of .O172 10- ohm-m. and the silverhas the approximate resistivity value of .0163 1()- ohm-m.

4. In a moving coil loudspeaker including a magnetic gap having atransverse flux density B, a moving coil structure comprising:

a voice coil winding,

a sound producing diaphragm,

and a constant velocity damping element interconnecting said winding andsaid diaphragm,

said element being composed of a material having a resistivity kdetermined by the relationship:

6 where; L is the mean cross-sectional length of said element takentransverse to the axis of said magnetic gap; L is the length of said gapparallel to the axis thereof; L :L sufiicient that at maximumdisplacement of the winding, the free end of the element will be beyondthe magnetic gap, and the element itself in the magnetic gap; T is themean cross-sectional thickness of said element measured transverse tothe said axis of said gap; M is the mass of said structure; w is theangular frequency of said structure at the point of natural resonancethereof; L is greater than L and where all of the above factors aremeasured in the MKS system of units. 5. In a moving coil loudspeaker, astructure according to claim 4, wherein:

the resistivity k determined by the said relationship is equal to .01610 ohm-meters. 6. In a moving coil loudspeaker, a structure according toclaim 3, wherein:

the said element is made'or" material selected from the group consistingof copper and silver. 7. In a moving coil loudspeaker having an annularmagnetic gap, a structure according to claim 4 wherein: the said elementis a split-cylinder form on which said winding is wound. 8. In a movingcoil loudspeaker having an annular magnetic gap, a structure accordingto claim 4 wherein: the said element is a closed cylinder form on whichsaid winding is wound. 9. In a moving coil loudspeaker having an annularmagnetic gap, a structure according to claim 4 wherein: the said elementis a cylindrical form, fashioned out of a slightly oblique parallelogramof the said element material, on which the said winding is wound.

References Cited by the Examiner UNITED STATES PATENTS 1,767,837 6/30Davis et al. 179-1155 1,971,452 8/34 Herrmann 179-1155 2,590,554 3/52Lukacs 179-1155 2,769,942 11/56 Hasson 179-1155 2,897,291 7/59 Burke179-1155 ROBERT H. ROSE, Primary Examiner.

1. IN A MOVING COIL LOUDSPEAKER, A MOVING COIL STRUCTURE COMPRISING: (A)A FORM DEFINING A CONSTANT VELOCITY DAMPING ELEMENT COMPOSED OF MATERIALSELECTED FROM THE GROUP CONSISTING OF RELATIVELY PURE COPPER ANDRELATIVELY PURE SILVER, TO BE POSITIONED IN A MAGNET GAP, (B) A VOICECOIL WINDING ON THE FORM, (C) A MAGNET HAVING INNER AND OUTER POLE PIECEEXTENSIONS DISPOSED TO DEFINE A MAGNETIC GAP TO RECEIVE BETWEEN THEINNER AND OUTER POLE PIECE EXTENSIONS THE VOICE COIL AND FORM, (D) ASOUND PRODUCING DIAPHRAGM CONNECTED TO ONE END OF THE FORM, (E) THE FORMLONGITUDINALLY DIMENSIONED SUFFICIENTLY GREATER THAN THE LONGITUDINALDIMENSION OF THE GAP SO THAT THE FREE OTHER END OF THE FORM WILL BEBEYOND THE MAGNETIC GAP AT MAXIMUM DISPLACEMENT OF THE VOICE COIL.